Understanding Phreatic Eruptions- Recent Observations of Kusatsu-Shirane Volcano and Equivalents -Editorial Board (October 2023)
Editor-in-Chief
Takeshi Sagiya, Nagoya University, Japan

Vice Editors-in-Chief
Masahito Nosé, Nagoya City University, Japan

Aitaro Kato, University of Tokyo, Japan

Editors
Koki Aizawa
Hideo Aochi
Pascal Audet
Kiyoshi Baba
Toshitaka Baba
Joseph B.H. Baker
Nanan Balan
Stephan Christoph Buchert
Sonia Calvari
Xiaowei ChenPeter Chi
Kuo-En Ching
Phil Cummins
Hao Dong
Guangyu Fu
Hidenori Genda
Aditya Gusman
Stuart Henrys
Yuji Himematsu

Hauke Hussmann
Mie Ichihara
Hidekatsu Jin
Art Jolly
Shunichi Kamata
Nobuki Kame
Katsuya Kaneko
Ikuo Katayama
Yoko Kebukawa
Kunihiro Keika
Kostas Konstantinou
Alexey Kuvshinov
Juanjo Ledo
Koji Matsuo
Yasunobu Miyoshi
Atsuko Namiki
Yasuhito Narita
Kazunari Nawa
Kiwamu Nishida

Hiroyo Ohya
Duggirala Pallamraju
Severine Rosat
Susumu Saito
David Shelly
Hisayoshi Shimizu
Wataru Suzuki
Youichiro Takada
Sunny Tam
Naoki Uchida
Yanbin Wang
Yih-Min Wu
Takahiro Yamamoto
Yuhji Yamamoto
Yusuke Yokota
Yohei Yukutake
Seiji Zenitani

Hiroo Kanamori
Jun'ichiro Kawaguchi
Takafumi Matsui
Hitoshi Mizutani

Yasuo Ogawa
Toshitaka Tsuda
Zhongliang Wu
Kiyoshi Yomogida

Advisory Board
Benjamin Fong Chao
Bernard Chouet
Yoshimori Honkura
Toshihiko Iyemori

Journal Scope
Earth, Planets and Space (EPS) is the official journal of Society of Geomagnetism and Earth, Planetary
and Space Sciences, The Seismological Society of Japan, The Volcanological Society of Japan, The
Geodetic Society of Japan, and The Japanese Society for Planetary Sciences.
EPS is a peer-reviewed, open-access journal published under SpringerOpen. It is an international journal
covering scientific articles in the entire field of earth and planetary sciences, particularly geomagnetism,aeronomy, space science, seismology, volcanology, geodesy, and planetary science. EPS also welcomesarticles in new and interdisciplinary subjects, and technical reports on instrumentation and software.
The journal was launched in 1998 and has since published over 4200 articles. All of them are available forfree on SpringerLink:
https://link.springer.com/journal/40623/volumes-and-issues
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Submit your next research article to Earth, Planets and Space via:
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Yours sincerely,
Prof. Takeshi Sagiya
Editor-in-Chief, Earth, Planets and Space
eic@earth-planets-space.org

Earth, Planets and Space(2022) 74:100
Ogawa et al. Earth, Planets and Space
https://doi.org/10.1186/s40623-022-01643-0

Open Access

PREFACESpecial issue “Understanding
phreatic eruptions - recent observationsof Kusatsu-Shirane volcano and equivalents -”Yasuo Ogawa1*, Takeshi Ohba2, Tobias P. Fischer3, Mare Yamamoto4 and Art Jolly5,6Introduction
The phreatic eruption is one of the volcanic eruptionstyles triggered by the rapid vaporization of heated fluids at shallow depth without the involvement of juvenile
magma (e.g., Stix and deMoor 2018). Although it is arelatively small-scale near-surface phenomenon, its precursors are difficult to detect. Thus, for the mitigation of
volcanic risk, it is an important research topic. This special issue is dedicated to further understanding phreatic
eruptions, notably the phreatic eruption of the KusatsuShirane volcano in 2018. This issue also includes papers
on the phreatic eruptions of similar volcanoes in Japanand the world.

approaches (Nurhasan et al. 2006; Takahashi and Fujii
2014; Tseng et al. 2020).
In January 2018, an unexpected phreatic eruption(VEI = 1) took place at the three vents of the Moto-Shirane pyroclastic cone after 1500 years of dormancy. It
occurred near the skiing slope in winter. One person was
killed, and 11 people were injured. This event revealed
the difficulty of the prediction of phreatic eruptions, inparticular for dormant volcanoes, and motivated longterm and more regional-scale studies of the dormant
volcanoes. This special issue includes past and ongoing
research on the Kusatsu-Shirane volcano with various
disciplines such as seismology, geodesy, geomagnetism,
geochemistry, petrology, and risk assessment. We also
included papers on other similar volcanoes with phreatic eruption activities. We hope these papers will havefuture impact on studies of short-term and long-term
assessments of volcanic risk, particularly for phreatic
eruptions, and will lead to mitigation of volcanic risk. We
briefly introduce summaries of the contributed papers as
follows.
Kametani et al. (2021) reported the total mass ejection
from the 2018 phreatic eruption at the Moto-Shirane
pyroclastic cone. They mapped the volcanic ejecta fromthe three craters which opened during the 2018 eruption. The volcanic blocks were distributed within 500 m
from the new craters. On the other hand, the ash reached
25 km from the craters. The total ejected mass was estimated as 2.4 × 107–3.4 × 107 kg.
Sato (2021) reported that the three radars of the Japan
Metrological Agency (JMA) captured the 2018 eruption. The radar echoes were detected in the lower andmiddle troposphere, the plume height was estimatedPapers on the Kusatsu‑Shirane volcano
Kusatsu-Shirane volcano is an active dacite-andesite
Quaternary volcano located in Gunma Prefecture, Central Japan. From north to south, the volcano consists ofthree pyroclastic cones, Shirane, Ainomine, and MotoShirane. Since 1805, its volcanic activity has been characterized by phreatic eruptions, particularly around the
Yugama crater of the Shirane pyroclastic cone (Terada2018). Therefore, multi-disciplinary studies have been
conducted around the most active Yugama crater tomitigate volcanic hazards using geochemical (Ohbaet al. 1994, 2019; Terada et al. 2018), seismological(Nakano et al. 2003; Mori et al. 2006), and geomagnetic*Correspondence: oga@ksvo.titech.ac.jp
1

Volcanic Fluid Research Center, Tokyo Institute of Technology, Tokyo, Japan
Full list of author information is available at the end of the article© The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://creativecommons.org/licenses/by/4.0/.

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Understanding Phreatic Eruptions - Recent Observations of Kusatsu-Shirane Volcano and Equivalents Ogawa et al. Earth, Planets and Space(2022) 74:100

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post-deformation due to the 2011 Tohoku earthquake,the deformation around the Kusatsu-Shirane volcano wasinferred and modeled as a spheroidal pressure source
model located at 4 km below the surface. A volume
change of the spheroid expressed the long-term deformation. The 2014 unrest at Yugama crater and the 2018
eruption at Moto-Shirane cone were characterized bysharp increases in the spheroid volume, implying an
increase of magmatic input. This study was successful indetecting long-term precursory volcanic activity.
For a further general understanding of the KusatsuShirane volcano system, magnetotelluric surveys havebeen undertaken. Tseng et al. (2020) reported thethree-dimensional resistivity structure surrounding the
Yugama crater. They imaged a conductive clay cap layerthat forms an impermeable seal for the geothermal system under the Yugama crater. This capping structure can
explain the upper limit of the micro-seismicity, demagnetization source locations, and the 2014 inflation source
locations. They also found a deep conductor which
implies high-salinity supercritical fluid below the microseismicity cutoff, presumably capped by a silica seal .
Koyama et al. (2021) reported the aeromagnetic survey
over the Kusatsu-Shirane volcano after the 2018 eruption
using an unmanned helicopter. The equipment has anadvantage for the high spatial resolution because of thedata acquisition at the low altitude from the surface andthe safe measurement when the volcanic target is difficult
to access. They modeled the three-dimensional distribution of magnetic intensity. The recent volcanic deposits
showed the surface positive magnetic intensity, and theunderlying negative intensity was interpreted as an older
lava flow. This measurement can be baseline data for
future repeat measurements for detection of temporal
temperature change of the volcano.
Ueki et al. (2020) reported the petrological investigations on the orthopyroxene and magnetite symplectites
associated with olivine in the Sessho lava, which erupted
about 3000 years ago (e.g., Terada 2018). The varieties ofthe symplectites suggest that the recharge of the basaltic magma into the existing magma reservoir repeatedly
occurred under the Kusatsu-Shirane volcano.

as 5580 m, and the total ejected mass was estimated as
6.7 × 107–6.5 × 108 kg, depending on the chosen empirical models for magmatic eruptions.
After the unexpected eruption in winter, hazard assessment is needed to estimate the potential riskof snow-related lahars immediately. Kataoka et al. (2021)
demonstrated numerical lahar flow simulations using the
Titan2D by considering proximal tephra deposits andsnow surveys. They made three lahar scenarios, including
rain-on-snow, ice/snow slurry, and total snowmelt triggered by a new eruption, and showed the potential flow
paths and travel distances.
The observed precursors for the 2018 Kusatsu-Shirane
volcanic eruption were limited. One of the reasons might
be that the seismic and geodetic networks were sparsefor the dormant Moto-Shirane pyroclastic cone, compared with those around the active Yugama crater at
Shirane pyroclastic cone. However, the seismic network
detected the precursory signals just two minutes before
the eruption. Yamada et al. (2021) investigated the temporal changes of the 5–20 Hz tremor amplitude observedby the seismic stations and located the tremor source.
The result showed that the tremor source migrated for
1 km horizontally in two minutes before the eruption
and finally reached the eruption place at 0.5–1 km depth
from the surface. The pathway of tremor source differsfrom the known seismicity around the active Yugama
crater, suggesting a unique migration of fluid movement.
Himematsu et al. (2020) analyzed the L-band satellite
synthetic aperture radar (SAR) data, pertaining to the2018 Kusatsu-Shirane volcanic eruption, searching forprecursory signals of phreatic eruptions. They could findno precursors, but they detected co- and post-eruptive
deformations around the new craters. They detectedcombinations of normal faulting and left-lateral slip for
co-eruptive deformation followed by an isotropic deflation. They interpreted that the co-eruptive fault plane canbe a pathway of volcanic fluid from the reservoir imagedby magnetotellurics (Matsunaga et al. 2020).
Terada et al. (2021) analyzed tilt data obtained at thethree borehole stations surrounding the Yugama crater
during the 2018 Moto-Shirane cone eruption. The tilt
record showed inflation stating two minutes prior to theeruption, which is consistent with the initiation of thetremor. Deflation was recorded after the eruption. The
tilt data were modeled using a sub-vertical crack for inflation and deflation phases. Inflation/deflation volume wasestimated as 5.1 × 105 and 3.6 × 105 m3, respectively. The
total heat discharge was estimated as > 1014 J, equivalent
to annual heat discharge from the active Yugama crater.
Munekane (2021) reported long-term volcanic deformation using GNSS data around the Kusatsu-Shirane
volcano. After carefully removing the regional-scale

Papers on similar volcanoes in Japan and the world
This special issue also includes studies on phreatic eruptions at other volcanoes in Japan, New Zealand, andCosta Rica, and the summaries are described below.
Ichiki et al. (2021) obtained wideband magnetotelluric data around the Azumayama volcano,northeastern Japan, and imaged the magmatic-hydrothermal system from the three-dimensional inversion. They detected a conductor (less than 3 Ωm) at
3–15 km below sea level. The conductors imply the2Earth, Planets and Space
Ogawa et al. Earth, Planets and Space(2022) 74:100

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general scheme for forecasting phreatic eruptions using
data from the continuous seismic records.
Park et al. (2020) analyzed very long-period earthquakes (VLPs) from 2007 to the end of 2019 at
Whakaari/White Island volcano, New Zealand. The
waveform shows similitudes, implying that the source
locations and the mechanism do not change. From thesemblance analyses, two families are detected and characterized by the mirror image, but they occur in a different stage of volcanic activities. The one is stable over
the whole period, while the other occurs as swarms thatmark the onset of phreatic activity.
Melchor et al. (2020) analyzed volcanic tremors during the 2012 and 2013 phreatic eruptions at Copahue
volcano, southern Andes. They could discriminate thetremor signals from the noise by the lower permutation
entropies and higher degrees of polarizations even if
the signal-to-noise level is low.
Rouwet (2021) presented the first geochemical modelof the Turrialba and the Irazú volcanoes in Costa Rica.
The Turrialba volcano became active in 2004 after
140 years of dormancy. After the onset of the 2010
phreatic eruption of the Turrialba volcano, the underlying geothermal system changed as seen from the significant increase in the fumarole output at the Turrialba
volcano. At the same time, the crater lake at the inactive Irazú volcano disappeared.

hydrothermal fluid and the water-saturated andesitic
melt. The fact that the location of the Mogi inflation
source coincides with the top of the conductor implies
that the percolation threshold governs the inflation.
Mannen et al. (2021) describe the recent reactivationof the Hakone volcano after the 2015 phreatic eruption(Mannen et al. 2018). After the eruption, they also see
deep inflation at 10 km depth, and deep low-frequency
earthquakes, which are interpreted as the re-supply ofmagma and magmatic fluids. While the 2015 eruption
center appears to have lower seismicity at present, theseismic swarm area has shifted to the rim of the caldera.
The post-eruption activity suggests that the system has
again sealed and phreatic eruptions are possible.
Ohba et al. (2021) analyzed the fumarolic gases at
Kirishima volcano, Kyushu, Japan, during the 2018 Ebinokogen Ioyama eruption. Sharp increases of SO2 andH2 concentrations were observed in 2017 and 2018prior to the phreatic eruption of April 2018. Oxygenand hydrogen isotope studies reveal the mixing of magmatic gases and meteoric water. Furthermore, theyfound that the high apparent equilibrium temperaturefrom SO2, H2S, H2, and H2O, together with low CO2/SO2 and H2S/SO2 ratios, can be used as precursor signals to the phreatic eruption.
Muramatsu et al. (2021) installed two infrasoundrecords and cameras near the two craters at Kirishima
volcano during the 2018 Ebinokogen Ioyama eruption. They identified the intense eruption with low-frequency infrasonic signals several hours after the onsetof the phreatic eruption.
Kurokawa and Ichihara (2020) measured infrasoundand seismic tremor at a single station during the 2013
and 2018 events at the Ioto island, an active volcano
located 1200 km south of Tokyo in the Izu–Bonin arc.
They could successfully identify the phreatic eruptionof the 2013 events by using spectral amplitude ratios ofthe vertical ground motion to the pressure oscillation.
However, for the 2018 event, the phreatic event was notclear. These differences imply that the differences of the
explosive nature may depend on whether the eruption
took place on land or underwater.
Caudron et al. (2021) reviewed the 15-year-longseismic data of the Whakaari White Island volcano, afrequently active volcano located 50 km off the northern coast of the North Island in New Zealand. They
focused on the ambient noises and tremors for the different activity periods of quiescence, unrest, magmaticand phreatic eruptions. Time and frequency evolutionof the volcanic tremor was monitored for 15 years by
Displacement Seismic Amplitude Ratio (DSAR), relative seismic velocity (dv/v), decorrelation, and the LuniSeismic Correlation (LSC). They finally proposed aConcluding remarks
The short-term prediction of phreatic eruptions
remains challenging because of the typically subtle and short-term precursors. As an example, the2018 Kusatsu-Shirane volcanic eruption at Moto-Shirane cone had precursory tremor and simultaneous
tilt changes just two minutes prior to eruption after
1500 years of dormancy. There are successful studies
on long-term assessment of phreatic eruptions, using
seismology, geodesy, and geochemistry, as shown in
the papers in this issue. Predicting phreatic eruptions
needs a basic understanding of the architecture and the
dynamics of magma–hydrothermal systems over multiple timescales, understanding that requires the use of
dense near-field milti-disciplinary monitoring.
The guest editors hope that the special issue papers
will promote further studies on phreatic eruptions
toward the mitigation of phreatic eruption risk.
Acknowledgements
We acknowledge all the contributing authors and reviewers. We also thank
Stephanie Prejean and Takeshi Sagiya for reviewing this preface article.

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Author contributions
YO drafted the manuscript, and all the co-authors contributed to the editing. All agreed with the contents. All authors read and approved the final
manuscript.

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injection of the 2018 phreatic eruption at Kusatsu-Shirane volcano with

Funding
Some of the published papers were supported by KAKENHI 17K20141 and
20H01992.
Availability of data and materials
Not applicable.

Declarations
Ethics approval and consent to participate
Not applicable.
Competing interests
All the authors have no competing interests.
Author details
1
Volcanic Fluid Research Center, Tokyo Institute of Technology, Tokyo, Japan.
2
Department of Chemistry, School of Science, Tokai University, Hiratsuka,
Japan. 3 Department of Earth and Planetary Sciences, University of New
Mexico, Albuquerque, NM, USA. 4 Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University,
Sendai, Japan. 5 GNS Science, Lower Hutt, New Zealand. 6 Present Address: US
Geological Survey, Hawaiian Volcano Observatory, Hawaii 96720, USA.
Received: 22 April 2022 Accepted: 22 April 2022

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Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

5Understanding Phreatic Eruptions - Recent Observations of Kusatsu-Shirane Volcano and Equivalents -FRONTIER LETTER

Open AccessA quest for unrest in multiparameter observations at
Whakaari/White Island volcano, New Zealand 2007–2018
Corentin Caudron*, Társilo Girona, Arthur Jolly, Bruce Christenson, Martha Kane Savage,
Roberto Carniel, Thomas Lecocq, Ben Kennedy, Ivan Lokmer, Alexander Yates, Ian Hamling,
Iseul Park, Geoff Kilgour and Agnès Mazot
Earth, Planets and Space 2021, 73:195 DOI: 10.1186/s40623-021-01506-0
Received: 24 February 2021, Accepted: 20 August 2021, Published: 21 October 2021

Abstract
The Whakaari/White Island volcano, located ~ 50 km off the east coast of the North Island in New Zealand, has experienced
sequences of quiescence, unrest, magmatic and phreatic eruptions over the last decades. For the last 15 years, seismic data
have been continuously archived providing potential insight into this frequently active volcano. Here we take advantage ofthis unusually long time series to retrospectively process the seismic data using ambient noise and tremor-based
methodologies. We investigate the time (RSAM) and frequency (PowerSpectral Density) evolution of the volcanic tremor, then estimate thechanges in the shallow subsurface using the Displacement Seismic
Amplitude Ratio (DSAR), relative seismic velocity (dv/v) and decorrelation,and the Luni-Seismic Correlation (LSC). By combining our new set ofobservations with the long-term evolution of earthquakes, deformation,
visual observations and geochemistry, we review the activity of Whakaari/White Island between 2007 and the end of 2018. Our analysis reveals theexistence of distinct patterns related to the volcano activity with periods ofcalm followed by cycles of pressurization and eruptions. We finally put
these results in the wider context of forecasting phreatic eruptions using
continuous seismic records.

Keywords: Volcanology, Monitoring, Phreatic eruption, Seismic noise, Tremor

Graphical abstract*Corresponding author: Corentin Caudron, corentin.caudron@univ-smb.fr

FULL PAPER

Open Access

Oxidation during magma mixing recorded by symplectites
at Kusatsu–Shirane Volcano, Central Japan
Kenta Ueki*, Mutsuko Inui, Kenta Matsunaga, Naoya Okamoto and Kazuki Oshio
Earth, Planets and Space 2020, 72:68 DOI: 10.1186/s40623-020-01192-4
Received: 11 February 2020, Accepted: 4 May 2020, Published: 13 May 2020

Abstract
Kusatsu–Shirane Volcano is an active Quaternary andesiticto-dacitic volcano located in the Central Japan Arc. We
conducted a detailed petrological investigation oforthopyroxene (opx)–magnetite (mt) symplectites
associated with olivine in the Sessho lava, an andesitic lava
flow from Kusatsu–Shirane. We concluded that the
symplectites are pseudomorphs after olivine and wereformed through the breakdown of olivine in a mafic magma
as a result of oxidation during mixing with a felsic magma.
Various olivines and opx–mt symplectites that show
different stages of the progressive breakdown reaction ofolivine coexist in a single lava flow. We suggest that basaltic
recharge into the magma reservoir beneath Kusatsu–Shirane
occurred repeatedly, leading to a hybrid andesite magmawith different types of olivine and symplectite being
erupted at Kusatsu–Shirane Volcano.

Keywords: Kusatsu–Shirane, Andesite, Symplectite, Magmamixing, Olivine, Active volcano

Graphical abstract*Corresponding author: Kenta Ueki, kenta_ueki@jamstec.go.jp
6Earth, Planets and Space

FULL PAPER

Open AccessClassification of long-term very long period (VLP) volcanic
earthquakes at Whakaari/White Island volcano, New ZealandIseul Park*, Arthur Jolly, Ivan Lokmer and Ben Kennedy
Earth, Planets and Space 2020, 72:92 DOI: 10.1186/s40623-020-01224-z
Received: 9 April 2020, Accepted: 29 June 2020, Published: 6 July 2020

Abstract
We have observed very long period earthquakes (VLPs) over the period 2007 to the end of 2019 at Whakaari/White Island
volcano, New Zealand. The earthquakes exhibit similitude between waveforms which suggests repeating source locations andprocesses. VLPs recorded at two permanent stations were detected using waveform semblance and were then classified into two
main families (F1 and F2) using a clustering analysis. The two families are characterized by ‘mirror image’ reverse waveformpolarity suggesting that they are genetically related, but occur during different evolutionary phases of volcanic activity. F1 eventsoccurred throughout the observation period, while F2 events mainly occurred as swarms that mark the onset of volcanic unrest.
A detailed cluster analysis reveals possible sub-families implying slight temporal evolutions within a family. Our results add to ourunderstanding of the volcanic magma–hydrothermal system at Whakaari/White Island indicating that relatively stable VLP
sources may be exploited to improve monitoring for
future unrest.

Keywords: VLP, Earthquake family, Swarm, Volcanic
unrest, Clustering analysis

Graphical abstract*Corresponding author: Iseul Park, i.park@gns.cri.nz

FULL PAPER

Open Access

Identification of infrasonic and seismic components of tremorsin single-station records: application to the 2013 and 2018
events at Ioto Island, Japan
Aika K. Kurokawa* and Mie Ichihara
Earth, Planets and Space 2020, 72:171 DOI: 10.1186/s40623-020-01302-2
Received: 14 June 2020, Accepted: 22 October 2020, Published: 9 November 2020

Abstract
Infrasonic stations are sparse at many volcanoes, especially those on remote islands and thosewith less frequent eruptions. When only a single infrasound station is available, the seismic–
infrasonic cross-correlation method has been used to extract infrasound from wind noise.
However, it does not work with intense seismicity and sometimes mistakes ground-toatmosphere signals as infrasound. This paper proposes a complementary method to identify
the seismic component and the infrasonic component using a single microphone and aseismometer. We applied the method to estimate the surface activity on Ioto Island. We
focused on volcanic tremors during the phreatic eruption on April 11, 2013, and during an
unconfirmed event on September 12, 2018. We used the spectral amplitude ratios of thevertical ground motion to the pressure oscillation and compared those for the tremors withthose for known signals generated by volcano-tectonic earthquakes and airplanes flying overthe station. We were able to identify the infrasound component in the part of the seismic
tremor with the 2013 eruption. On the other hand, the tremor with the unconfirmed 2018
event was accompanied by no apparent infrasound. We interpreted the results that the
infrasound with the 2013 event was excited by the vent opening or the ejection of ballistic
rocks, and the 2018 event was not an explosive eruption either on the ground or in the shallow
water. If there was any gas (and ash) emission, it might have occurred gently undersea. As themethod uses the relative values of on-site records instead of the absolute values, it is available
even if the instrument sensitivity and the station site effects are poorly calibrated.

Keywords: Volcanic tremor, Infrasound, Vertical ground motion, Spectral ratio, Ioto Island*Corresponding author: Aika K. Kurokawa, kurokawa@bosai.go.jp
7Graphical abstract

Understanding Phreatic Eruptions - Recent Observations of Kusatsu-Shirane Volcano and Equivalents -FULL PAPER

Open Access

Locating hydrothermal fluid injection of the 2018 phreatic eruption
at Kusatsu-Shirane volcano with volcanic tremor amplitude
Taishi Yamada*, Aika K. Kurokawa, Akihiko Terada, Wataru Kanda, Hideki Ueda, Hiroshi Aoyama,
Takahiro Ohkura, Yasuo Ogawa and Toshikazu Tanada
Earth, Planets and Space 2021, 73:14 DOI: 10.1186/s40623-020-01349-1
Received: 9 September 2020, Accepted: 24 December 2020, Published: 11 January 2021

Abstract
Kusatsu-Shirane volcano hosts numerous thermal springs, fumaroles, and the crater lake of Yugama.
Hence, it has been a particular study field for hydrothermal systems and phreatic eruptions. On 23
January 2018, a phreatic eruption occurred at the Motoshirane cone of Kusatsu-Shirane, where no
considerable volcanic activity had been reported in observational and historical records. To
understand the eruption process of this unique event, we analyzed seismic, tilt, and infrasoundrecords. The onset of surface activity accompanied by infrasound signal was preceded by volcanic
tremor and inflation of the volcano for ~ 2 min. Tremor signals with a frequency band of 5–20 Hz
remarkably coincide with the rapid inflation. We apply an amplitude source location method toseismic signals in the 5–20 Hz band to estimate tremor source locations. Our analysis locates tremor
sources at 1 km north of Motoshirane and at a depth of 0.5–1 km from the surface. Inferred source
locations correspond to a conductive layer of impermeable cap-rock estimated by magnetotelluric
investigations. An upper portion of the seismogenic region suggests hydrothermal activity hosted
beneath the cap-rock. Examined seismic signals in the 5–20 Hz band are typically excited by volcanotectonic events with faulting mechanism. Based on the above characteristics and background, weinterpret that excitation of examined volcanic tremor reflects small shear fractures induced bysudden hydrothermal fluid injection to the cap-rock layer. The horizontal distance of 1 km between
inferred tremor sources and Motoshirane implies lateral migration of the hydrothermal fluid,
although direct evidence is not available. Kusatsu-Shirane has exhibited unrest at the Yugama lake
since 2014. However, the inferred tremor source locations do not overlap active seismicity beneath
Yugama. Therefore, our result suggests that the 2018 eruption was triggered by hydrothermal fluid
injection through a different pathway from that has driven unrest activities at Yugama.

a

b

c

d

Keywords: Kusatsu-shirane, Phreatic eruptions, Volcanic tremor, Hydrothermal system

Graphical abstract*Corresponding author: Taishi Yamada, yamada.taishi.8m@kyoto-u.ac.jp

FULL PAPER

Open Access

Volcanic unrest at Hakone volcano after the 2015 phreatic
eruption: reactivation of a ruptured hydrothermal system?
Kazutaka Mannen*, Yuki Abe, Yasushi Daita, Ryosuke Doke, Masatake Harada, George Kikugawa,
Naoki Honma, Yuji Miyashita and Yohei Yukutake
Earth, Planets and Space 2021, 73:80 DOI: 10.1186/s40623-021-01387-3
Received: 28 September 2020, Accepted: 22 February 2021, Published: 26 March 2021

Abstract
Since the beginning of the twenty-first century, volcanic unrest has occurred every 2–5 years at Hakone volcano. After the 2015 eruption, unrest activity
changed significantly in terms of seismicity and geochemistry. Like the pre- and co-eruptive unrest, each post-eruptive unrest episode was detected bydeep inflation below the volcano (~ 10 km) and deep low frequency events, which can be interpreted as reflecting supply of magma or magmatic fluid
from depth. The seismic activity during the post-eruptive unrest episodes also increased; however, seismic activity beneath the eruption center during theunrest episodes was significantly lower, especially in the shallow region (~ 2 km), while sporadic seismic swarms were observed beneath the caldera rim,
~ 3 km away from the center. This observation and a recent InSAR analysis imply that the hydrothermal systemof the volcano could be composed of multiple sub-systems, each of which can host earthquake swarms andshow independent volume changes. The 2015 eruption established routes for steam from the hydrothermal
sub-system beneath the eruption center (≥ 150 m deep) to the surface through the cap-rock, allowing
emission of super-heated steam (~ 160 ºC). This steam showed an increase in magmatic/hydrothermal gas
ratios (SO2/H2S and HCl/H2S) in the 2019 unrest episode; however, no magma supply was indicated by seismic
and geodetic observations. Net SO2 emission during the post-eruptive unrest episodes, which remained
within the usual range of the post-eruptive period, is also inconsistent with shallow intrusion. We consider thatthe post-eruptive unrest episodes were also triggered by newly derived magma or magmatic fluid from
depth; however, the breached cap-rock was unable to allow subsequent pressurization and intensive seismic
activity within the hydrothermal sub-system beneath the eruption center. The heat released from the newly
derived magma or fluid dried the vapor-dominated portion of the hydrothermal system and inhibited
scrubbing of SO2 and HCl to allow a higher magmatic/hydrothermal gas ratio. The 2015 eruption could havealso breached the sealing zone near the brittle–ductile transition and the subsequent self-sealing process
seems not to have completed based on the observations during the post-eruptive unrest episodes.

Keywords: Hakone volcano, Phreatic eruption, Hydrothermal system, Sealing zone, Cap-rock, Volcanic
unrest, Scrubbing of magmatic gas*Corresponding author: Kazutaka Mannen, mannen@onken.odawara.kanagawa.jp
8Graphical abstract

Earth, Planets and Space

FULL PAPER

Open Access

Behavior of magmatic components in fumarolic gases related to
the 2018 phreatic eruption at Ebinokogen Ioyama volcano,
Kirishima Volcanic Group, Kyushu, Japan
Takeshi Ohba*, Muga Yaguchi, Urumu Tsunogai, Masanori Ito and Ryo Shingubara
Earth, Planets and Space 2021, 73:81 DOI: 10.1186/s40623-021-01405-4
Received: 12 January 2021, Accepted: 24 March 2021, Published: 31 March 2021

Abstract
Direct sampling and analysis of fumarolic gas was conducted at Ebinokogen Ioyama volcano, Japan, between December 2015 andJuly 2020. Notable changes in the chemical composition of gases related to volcanic activity included a sharp increase in SO2 and H2concentrations in May 2017 and March 2018. The analyses in March 2018 immediately preceded the April 2018 eruption at Ioyama
volcano. The isotopic ratios of H2O in fumarolic gas revealed the process of formation. Up to 49% high-enthalpy magmatic vapor
mixed with 51% of cold local meteoric water to generate coexisting vapor and liquid phases at 100–160 °C. Portions of the vapor andliquid phases were discharged as fumarolic gases and hot spring water, respectively. The
CO2/SO2 ratio of the fumarolic gas was higher than that estimated for magmatic vapor due
to SO2 hydrolysis during the formation of the vapor phase. When the flux of the magmatic
vapor was high, effects of hydrolysis were small resulting in low CO2/SO2 ratios in fumarolicgases. The high apparent equilibrium temperature defined for reactions involving SO2, H2S,
H2 and H2O, together with low CO2/SO2 and H2S /SO2 ratios were regarded to be precursor
signals to the phreatic eruption at Ioyama volcano. The apparent equilibrium temperature
increased rapidly in May 2017 and March 2018 suggesting an increased flux of magmatic
vapor. Between September 2017 and January 2018, the apparent equilibrium temperature
was low suggesting the suppression of magmatic vapor flux. During this period, magmatic
eruptions took place at Shinmoedake volcano 5 km away from Ioyama volcano. We
conclude that magma sealing and transport to Shinmoedake volcano occurred
simultaneously in the magma chamber beneath Ioyama volcano.

Keywords: Phreatic eruption, Volcanic gas, Fumarole, Hydrothermal reservoir, Magmatic vapor

Graphical abstract*Corresponding author: Takeshi Ohba, takeshi_ohba@tokai-u.jp

FULL PAPER

Open AccessResponse of a hydrothermal system to escalating phreatic unrest:
the case of Turrialba and Irazú in Costa Rica (2007–2012)
D. Rouwet*, R. Mora-Amador, C. Ramírez, G. González, E. Baldoni, G. Pecoraino, S. Inguaggiato,
B. Capaccioni, F. Lucchi and C. A. Tranne
Earth, Planets and Space 2021, 73:142 DOI: 10.1186/s40623-021-01471-8
Received: 30 December 2020, Accepted: 24 June 2021, Published: 12 July 2021

Abstract
This study presents the first hydrogeochemical model of the hydrothermal systems of Turrialba and Irazú volcanoes in central Costa Rica,
manifested as thermal springs, summit crater lakes, and fumarolic degassing at both volcanoes. Our period of observations (2007–2012)
coincides with the pre- and early syn-phreatic eruption stages of Turrialba volcano that resumed volcanic unrest since 2004, after almost 140
years of quiescence. Peculiarly, the generally stable Irazú crater lake dropped its level during this reawakening of Turrialba. The isotopic
composition of all the discharged fluids reveals their Caribbean meteoric origin. Four groups of thermal springs drain the northern flanks of
Turrialba and Irazú volcanoes into two main rivers. Río Sucio (i.e. “dirty river”) is a major rock remover on the North flank of Irazú, mainly fed by
the San Cayetano spring group. Instead, one group of thermal springs discharges towards the south of Irazú. All thermal spring waters are of
SO4-type (i.e. steam-heated waters), none of the springs has, however, a common hydrothermal end-member. A water mass budget for thermal
springs results in an estimated total output flux of 187 ± 37 L/s, with 100 ± 20 L/s accounted for by
the San Cayetano springs. Thermal energy release is estimated at 110 ± 22 MW (83.9 ± 16.8 MW by
San Cayetano), whereas the total rock mass removal rate by chemical leaching is ~ 3000 m3/year(~ 2400 m3/year by San Cayetano-Río Sucio). Despite Irazú being the currently less active
volcano, it is a highly efficient rock remover, which, on the long term can have effects on thestability of the volcanic edifice with potentially hazardous consequences (e.g. flank collapse,
landslides, phreatic eruptions). Moreover, the vapor output flux from the Turrialba fumaroles
after the onset of phreatic eruptions on 5 January 2010 showed an increase of at least ~ 260 L/sabove pre-eruptive background fumarolic vapor fluxes. This extra vapor loss implies that thedrying of the summit hydrothermal system of Turrialba could tap deeper than previously
thought, and could explain the coincidental disappearance of Irazú’s crater lake in April 2010.

Keywords: Irazú and Turrialba volcanoes, Phreatic eruptions, Thermal springs, Fluid

geochemistry, Crater lake disappearance, Water budget analysis, Hazard
assessment*Corresponding author: D. Rouwet, dmitri.rouwet@ingv.it
9

Fig. 4

Understanding Phreatic Eruptions - Recent Observations of Kusatsu-Shirane Volcano and Equivalents -FULL PAPER

Open Access

Magma reservoir beneath Azumayama Volcano, NE Japan,
as inferred from a three-dimensional electrical resistivity
model explored by means of magnetotelluric method
Masahiro Ichiki*, Toshiki Kaida, Takashi Nakayama, Satoshi Miura, Mare Yamamoto, Yuichi Morita
and Makoto Uyeshima
Earth, Planets and Space 2021, 73:150 DOI: 10.1186/s40623-021-01451-y
Received: 29 December 2020, Accepted: 5 June 2021, Published: 27 July 2021

Abstract
An electrical resistivity model beneath Azumayama Volcano, NE Japan, is explored using magnetotelluric method to probe the magma/hydrothermal fluid
distribution. Azumayama is one of the most concerning active volcanoes capable of producing a potential eruption triggered by the 2011 Tohoku-Oki Earthquake.
The three-dimensional resistivity model reveals a conductive magma reservoir (< 3 Ωm) at depths of 3–15 km
below sea level (bsl). The 67% and 90% confidence intervals of resistivity are 0.2–5 Ωm and 0.02–70 Ωm,
respectively, for the magma reservoir. We assumed dacitic melt + rock at a shallow depth of 4 km bsl and andesitic
melt + rock at a greater depth of 9 km bsl. The confidence interval of resistivity cannot be explained by using dacitic
melt + rock condition at a depth of 4 km bsl. This suggests that very conductive hydrothermal fluids coexist withdacitic melt and rock in the shallow part of the magma reservoir. For the depth of 9 km bsl, the 67% confidence
interval of resistivity is interpreted as water-saturated (8.0 weight %) andesitic melt–mafic rock complex with melt
volume fractions greater than 4 volume %, while the shear wave velocity requires the fluid and/or melt volume
fraction of 6–7 volume % at that depth. Considering the fluid and/or melt volume fraction of 6–7 volume %, theconductive hydrous phase is likewise required to explain the wide range of the 67% confidence interval ofresistivity. The Mogi inflation source determined from geodetic data lies on the resistive side near the top boundaryof the conductive magma reservoir at a depth of 2.7 or 3.7 km bsl. Assuming that the resistivity of the inflation
source region is above the upper bound of the confidence interval of resistivity for the conductive magma reservoir
and that the source region is composed of hydrothermal fluid + rock, the resistivity of the source region is explainedby a hydrothermal fluid volume fraction below 5 volume %, which is the percolation threshold porosity in aneffusive eruption. This indicates that the percolation threshold characterizes the inflation source region.

Keywords: Magnetotellurics, Electrical resistivity, Shear wave velocity, Magma reservoir, Melt fraction,
Permeability, Percolation threshold, Hydrothermal fluid, Mogi inflation source

Graphical abstract*Corresponding author: Masahiro Ichiki, ichiki@tohoku.ac.jp

FULL PAPER

Open Access

The 2018 phreatic eruption at Mt. Motoshirane of Kusatsu–Shirane
volcano, Japan: eruption and intrusion of hydrothermal fluid
observed by a borehole tiltmeter network
Akihiko Terada*, Wataru Kanda, Yasuo Ogawa, Taishi Yamada, Mare Yamamoto, Takahiro Ohkura,
Hiroshi Aoyama, Tomoki Tsutsui and Shin’ya Onizawa
Earth, Planets and Space 2021, 73:157 DOI: 10.1186/s40623-021-01475-4
Received: 28 February 2021, Accepted: 1 July 2021, Published: 30 July 2021

Abstract
We estimate the mass and energy budgets for the 2018 phreatic eruption of Mt. Motoshirane on Kusatsu–Shirane volcano, Japan, based ondata obtained from a network of eight tiltmeters and weather radar echoes. The tilt records can be explained by a subvertical crack model.
Small craters that were formed by previous eruptions are aligned WNW–ESE, which is consistent
with the strike of the crack modeled in this study. The direction of maximum compressive stress inthis region is horizontal and oriented WNW–ESE, allowing fluid to intrude from depth through acrack with this orientation. Based on the crack model, hypocenter distribution, and MT resistivity
structure, we infer that fluid from a hydrothermal reservoir at a depth of 2 km below Kusatsu–
Shirane volcano has repeatedly ascended through a pre-existing subvertical crack. The inflation anddeflation volumes during the 2018 eruption are estimated to have been 5.1 × 105 and 3.6 × 105 m3,
respectively, meaning that 1.5 × 105 m3 of expanded volume formed underground. The total heatassociated with the expanded volume is estimated to have been ≥ 1014 J, similar to or exceeding theannual heat released from Yugama Crater Lake of Mt. Shirane and that from the largest eruption
during the past 130 year. Although the ejecta mass of the 2018 phreatic eruption was small, theeruption at Mt. Motoshirane was not negligible in terms of the energy budget of Kusatsu–Shirane
volcano. A water mass of 0.1–2.0 × 107 kg was discharged as a volcanic cloud, based on weather
radar echoes, which is smaller than the mass associated with the deflation. We suggest that
underground water acted as a buffer against the sudden intrusion of hydrothermal fluids, absorbingsome of the fluid that ascended through the crack.
10

10

20

0

10

0

20

Keywords: Phreatic eruption, Tiltmeter, Kusatsu–Shirane volcano, Hydrothermal system, Tensile
crack, Brittle–ductile transition zone

*Corresponding author: Akihiko Terada, terada@ksvo.titech.ac.jp
10

EW [km]Fig. 50Earth, Planets and Space

FULL PAPER

Open Access

Modeling long-term volcanic deformation at Kusatsu-Shiraneand Asama volcanoes, Japan, using the GNSS coordinate
time series
Hiroshi Munekane
Earth, Planets and Space 2021, 73:192 DOI: 10.1186/s40623-021-01512-2
Received: 15 April 2021, Accepted: 4 September 2021, Published: 19 October 2021

Abstract

Keywords: Kusatsu-Shirane volcano,

Asama volcano, Global
Navigation Satellite System,
Crustal deformation, Oblate
spheroid

-1
-2
-3
-4
-5
-6

2013 2014 2015 2016 2017 2018 2019 2020 2021

6

3

Volume change (10 m )Volume change (106m3)

63

Volume change (10 m )Long-term deformation of Kusatsu-Shirane and Asama volcanoes in central Japan were investigated using Global Navigation Satellite
System (GNSS) measurements. Large postseismic deformation caused by the 2011 Tohoku earthquake—which obscures the long-term
volcanic deformation—was effectively removed by approximating the postseismic and other recent tectonic deformation in terms ofquadrature of the geographical eastings/northings. Subsequently, deformation source parameters were estimated by the Markov Chain
Monte Carlo (MCMC) method and linear inversion, employing an analytical model that calculates the deformation from an arbitrary
oriented prolate/oblate spheroid. The deformation source of Kusatsu-Shirane volcano was found to be a sill-like oblate spheroid locateda few kilometers northwest of the Yugama crater at a depth of approximately 4 km, while that of Asama was also estimated to be asill-like oblate spheroid beneath the western flank of the edifice at a depth of approximately 12 km, along with the previously reported
shallow east–west striking dike at a depth of approximately 1 km. It was revealed that (1) volume changes of the Kusatsu-Shirane
deformation source and the shallow deformation source of Asama were correlated with the volcanic activities of the corresponding
volcanoes, and (2) the Asama deep source
Kusatsu source
Asama shallow source
Asama deep source
has been steadily losing volume, which may
0.2
10
3
0.15
2
indicate that the volcano will experience51
0.1
fewer eruptions in the near future.
0
0.05
0
-0.05
-0.1
-0.15
-0.2

2013 2014 2015 2016 2017 2018 2019 2020 2021

0

-5
-10
-15

2013 2014 2015 2016 2017 2018 2019 2020 2021

Graphical abstract

Corresponding author: Hiroshi Munekane, munekane-h96nu@mlit.go.jp

FULL PAPER

Open Access

Crisis hazard assessment for snow-related lahars froman unforeseen new vent eruption: the 2018 eruption ofKusatsu-Shirane volcano, Japan
Kyoko S. Kataoka*, Kae Tsunematsu, Takane Matsumoto, Atsushi Urabe and Katsuhisa Kawashima
Earth, Planets and Space 2021, 73:220 DOI: 10.1186/s40623-021-01522-0
Received: 17 June 2021, Accepted: 18 September 2021, Published: 11 December 2021

Abstract
Two-thirds of the 111 active volcanoes in Japan are covered with snow for several months during winter and demonstrate high hazard and risk potentials
associated with snow-related lahars during and after eruptions. On 23 January 2018, a sudden phreatic eruption occurred at the ski field on KusatsuShirane (Mt. Motoshirane) volcano, Japan. This new vent eruption from the snow-clad pyroclastic cone required forecasting of future snow-related lahars
and crisis hazards zonation of downslope areas including Kusatsu town, a popular tourist site for skiing and hot springs. In order to achieve a prompt
hazard assessment for snow-related lahars, a multidisciplinary approach was carried out involving characterization of proximal tephra deposits, snow
surveys, and numerical lahar flow simulations using the Titan2D model. To determine the input parameters for the flow model, the consideration of snow
water equivalent (SWE) immediately after the eruption (on 29 January) and in the post-eruptive period (on 12 March), was significant. In the case of
Kusatsu-Shirane volcano during the winter of 2018, linear relationships between altitude and SWE, obtained at different elevations, were used to estimate
the snow volume around the new vents. Several scenarios incorporating snow and snowmelt (water), with or without the occurrence of a new eruption,
were simulated for the prediction of future lahars. Three lahar scenarios were simulated, including A) rain-on-snow triggered, B) ice/snow slurry, and C) fullsnowmelt triggered by a new eruption, and indicated the flow paths (inundation areas) andTwo-phase, bed 25
Confluene with Yazawa-gawa R.
travel distances. These were useful for lahar hazard zonation and identification of potential
high-risk areas. Since the input parameters required for the Titan2D flow model can be
Furiko-zawa C.
NH292
relatively easily determined, the model was suitable for the 2018 eruption at Motoshirane
where historical and geological lahar records are not available for calibration. The procedureused in the study will enable rapid lahar prediction and hazard zonation at snow-cladMononuguno-ike P.
volcanoes. Further consideration for simulating a cohesive-type flow, which was predicted
Shimizu-sawa C.
Cable car terminalby the primary deposits containing large amounts of clay minerals and could not be
expressed in the Titan2D flow model, is necessary.
Reference points
Initial point

Keywords: Lahar, Flow simulation, Titan2D, Snow-clad volcano, Snow survey, Crisis
hazard map, Kusatsu-Shirane volcano

Roads/Trails

0.05

Flow
thickness
(m)

1.00

2 km

Graphical abstract*Corresponding author: Kyoko S. Kataoka, kataoka@gs.niigata-u.ac.jp
11

KusatsuTown

Understanding Phreatic Eruptions - Recent Observations of Kusatsu-Shirane Volcano and Equivalents -EXPRESS LETTER

Open Access

Coeruptive and posteruptive crustal deformation associatedwith the 2018 Kusatsu-Shirane phreatic eruption basedon PALSAR-2 time series analysis
Yuji Himematsu*, Taku Ozawa and Yosuke Aoki
Earth, Planets and Space 2020, 72:116 DOI: 10.1186/s40623-020-01247-6
Received: 20 May 2020, Accepted: 1 August 2020, Published: 10 August 2020

Abstract
Coeruptive deformation helps to interpret physical processes associated with volcanic eruptions. Because phreatic eruptions cause small, localized
coeruptive deformation, we sometimes fail to identify plausible deformation signals. Satellite synthetic aperture radar (SAR) data allow us to identify
extensive deformation fields with high spatial resolutions. Herein, we report coeruptive crustal deformation associated with the 2018 Kusatsu-Shirane
phreatic eruption detected by time series analyses of L-band satellite SAR (ALOS-2/PALSAR-2) data. Cumulative deformation maps derived from SAR timeseries analyses show that subsidence and eastward displacement dominate the southwestern side of an eruptive crater with a spatial extent ofapproximately 2 km in diameter. Although we were unable to identify any significant deformation signals before the 2018 eruption, posteruptive
deformation on the southwestern side of the crater has been ongoing until the end of 2019. This prolonged deformation implies the progression ofposteruptive physical processes within a confined hydrothermal system, such as volcanic fluid discharge, similar to the processes observed during the2014 Ontake eruption. Although accumulated snow and dense vegetation hinder the detection of deformation signals on Kusatsu-Shirane volcano using
conventional InSAR data, L-band SAR with various temporal baselines allowed us to successfully extract both coeruptive and posteruptive deformation
signals. The extracted cumulative deformation is well explained by a combinationof normal faulting with a left-lateral slip component along a southwest-dipping
fault plane and an isotropic deflation. Based on the geological background inwhich the shallow hydrothermal system develops across Kusatsu-Shirane volcano,the inferred dislocation plane can be considered as a degassing pathway from theshallow hydrothermal system to the surface due to the phreatic eruption. We
reconfirmed that SAR data are a robust tool for detecting coeruptive andposteruptive deformations, which are helpful for understanding shallow physicalprocesses associated with phreatic eruptions at active volcanoes.

Keywords: Satellite SAR, Phreatic eruption, Hydrothermal system,

Kusatsu-Shirane volcano, Crustal deformation, ALOS-2/PALSAR-2

Graphical abstract*Corresponding author: Yuji Himematsu, himematsu@bosai.go.jp

EXPRESS LETTER

Open AccessOn data reduction methods for volcanic tremor characterization:
the 2012 eruption of Copahue volcano, Southern Andes
Ivan Melchor*, Javier Almendros, Roberto Carniel, Kostas I. Konstantinou, Marcia Hantusch
and Alberto Caselli
Earth, Planets and Space 2020, 72:134 DOI: 10.1186/s40623-020-01270-7
Received: 25 June 2020, Accepted: 4 September 2020, Published: 14 September 2020

Abstract
Improving the ability to detect and characterize long-duration volcanic tremor is crucial to understand the long-term dynamics
and unrest of volcanic systems. We have applied data reduction methods (permutation entropy and polarization degree,
among others) to characterize the seismic wave field near Copahue
volcano (Southern Andes) between June 2012 and January 2013,
when phreatomagmatic episodes occurred. During the selectedperiod, a total of 52 long-duration events with energy above thebackground occurred. Among them, 32 were classified as volcanic
tremors and the remaining as noise bursts. Characterizing each
event by averaging its reduced parameters, allowed us to study therange of variability of the different events types. We found that,compared to noise burst, tremors have lower permutation entropies
and higher dominant polarization degrees. This characterization is asuitable tool for detecting long-duration volcanic tremors in theambient seismic wave field, even if the SNR is low.

Keywords: Volcanic tremor, Phreatic eruption, Data reduction

method, Polarization degree, Permutation entropy,
Copahue volcano*Corresponding author: Ivan Melchor, ifmelchor@unrn.edu.ar
12

Graphical abstract

Earth, Planets and Space

EXPRESS LETTER

Open Access

Anatomy of active volcanic edifice at the Kusatsu–Shirane
volcano, Japan, by magnetotellurics: hydrothermal
implications for volcanic unrests
Kuo Hsuan Tseng, Yasuo Ogawa*, Nurhasan, Sabri Bülent Tank, Naoto Ujihara, Yoshimori Honkura,
Akihiko Terada, Yoshiya Usui and Wataru Kanda
Earth, Planets and Space 2020, 72:161 DOI: 10.1186/s40623-020-01283-2
Received: 15 February 2020, Accepted: 28 September 2020, Published: 23 October 2020

Abstract
We aimed to perform three-dimensional imaging of the underlying geothermal system to a depth of 2 km using magnetotellurics(MT) at around the Yugama crater, the Kusatsu–Shirane Volcano, Japan, which is known to have frequent phreatic eruptions. We
deployed 91 MT sites focusing around the peak area of 2 km × 2 km with typical spacings of 200 m. The full tensor impedances andthe magnetic transfer functions were inverted, using an unstructured tetrahedral finite
element code to include the topographic effect. The final model showed (1) lowpermeability bell-shaped clay cap (C1) as the near-surface conductor, (2) brine reservoir
as a deep conductor (C3) at a depth of 1.5 km from the surface, and (3) a verticalconductor (C2) connecting the deep conductor to the clay cap which implies anestablished fluid path. The columnar high-seismicity distribution to the east of the C2
conductor implies that the flushed vapor and magmatic gas was released from the brinereservoir by breaking the silica cap at the brittle–ductile transition. The pastmagnetization/demagnetization sources and the inflation source of the 2014 unrest arelocated just below the clay cap, consistent with the clay capped geothermal model
underlain by brine reservoir. The resistivity model showed the architecture of themagmatic–hydrothermal system, which can explain the episodic volcanic unrest.

Keywords: Phreatic eruption, Magnetotelluric method, Clay cap, Brine, Unrest
Graphical abstract*Corresponding author: Yasuo Ogawa, oga@ksvo.titech.ac.jp

EXPRESS LETTER

Open Access

Reconstructing surface eruptive sequence of 2018 small phreatic
eruption of Iwo-yama volcano, Kirishima Volcanic Complex,
Japan, by infrasound cross-correlation analysis
Dan Muramatsu*, Takeshi Matsushima and Mie Ichihara
Earth, Planets and Space 2021, 73:8 DOI: 10.1186/s40623-020-01344-6
Received: 30 October 2020, Accepted: 21 December 2020, Published: 5 January 2021

Abstract
The Iwo-yama volcano of the Kirishima Volcanic Complex in Japan had a small phreatic eruption in April 2018, which newly
formed multiple vents. The activity was recorded by two infrasound sensors and two monitoring cameras, which had been
installed within 1 km of the vents. This study identified infrasonic signals from the multiple vents by a cross-correlation analysis
between the two infrasound sensors. The analysis successfully revealed the signals from two main eruption craters andconstrained the infrasound onsets at the individual vents inthe two craters. We combined the infrasound results with
the images from the cameras and reconstructed thesequence of the small phreatic eruption of Iwo-yama. At
each of the two craters, the intense eruption, which was
depicted by the evident infrasound signals, occurred
several hours after the eruption onset. This study provides
a sequence of the activities of the multiple vents in aphreatic eruption, which will be useful for understanding
the phreatic eruption and hazard assessments.

Keywords: Volcano infrasound, Phreatic eruption,

Multiple-vent activities, Cross-correlation
analysis, Iwo-yama volcano, Monitoring

Fig. 5*Corresponding author: Dan Muramatsu, muramatsu@sevo.kyushu-u.ac.jp
13Understanding Phreatic Eruptions - Recent Observations of Kusatsu-Shirane Volcano and Equivalents -EXPRESS LETTER

Open Access

Kusatsu-Shirane volcano eruption on January 23, 2018,
observed using JMA operational weather radars
Eiichi Sato
Earth, Planets and Space 2021, 73:117 DOI: 10.1186/s40623-021-01445-w
Received: 31 December 2020, Accepted: 24 May 2021, Published: 31 May 2021

Abstract
A phreatic eruption suddenly occurred at Motoshirane (Kusatsu-Shirane
volcano, Japan) at 10:02 JST on January 23, 2018. A member of the Japan
Self-Defense Force was killed by volcanic blocks during training in
Motoshirane, and 11 people were injured by volcanic blocks or fragmentsof broken glass. According to a field survey, ash fall was confirmed in
Minakami, about 40 km east-northeast from Motoshirane. Although theeruption was not captured by a distant camera, the eruption plume/
cloud was captured by three of the Japan Meteorological Agency’s
operational weather radars. These radars observed the echo propagatedto the northeast in the lower troposphere, and to the east in the middle
troposphere. This is generally consistent with the observed ash fall
distribution. Using the modified probabilistic estimation method, themaximum plume height was estimated to be about 5580 ± 506 m (1σ)
above sea level. Estimates of the erupted mass based on the range ofplume heights from radar observations and the duration of volcanic
tremor during the eruption (about 8 min) do not match that obtained
from a field survey (3.0–5.0 × 107 kg). This discrepancy confirms that
estimates of erupted mass based on plume heights must account for
eruption style parametrically, which can only be constrained by casestudies of varied eruption styles.

Keywords: Weather radar, Remote sensing, Volcanic plume,
Eruption cloud, Kusatsu-shirane volcano

Graphical abstract

Corresponding author: Eiichi Sato, esato@mri-jma.go.jp

EXPRESS LETTER

Open Access

Aeromagnetic survey in Kusatsu-Shirane volcano, central
Japan, by using an unmanned helicopter
Takao Koyama*, Wataru Kanda, Mitsuru Utsugi, Takayuki Kaneko, Takao Ohminato,Atsushi Watanabe, Hiroshi Tsuji, Taro Nishimoto, Alexey Kuvshinov and Yoshiaki Honda
Earth, Planets and Space 2021, 73:139 DOI: 10.1186/s40623-021-01466-5
Received: 31 December 2020, Accepted: 21 June 2021, Published: 6 July 2021

Abstract

Elevation [m]Kusatsu-Shirane volcano is one of the active volcanoes in Japan. Phreatic explosions occurred in Mt. Shirane in 1983 and mostrecently, in 2018, in Mt. Motoshirane. Information on the subsurface structure is crucial for understanding the activity of
volcanoes with well-developed hydrothermal systems where phreatic eruptions occur. Here, we report aeromagnetic surveys
conducted at low altitudes using an unmanned helicopter. The survey aimed to obtain magnetic data at a high spatial
resolution to map the magnetic anomaly and infer the magnetization intensity distribution in the region immediately after
the 2018 Mt. Motoshirane eruption. The helicopter used in the survey was YAMAHA FAZER R G2, an autonomously driven
model which can fly along a precisely programmed course. The flight height above the ground and a measurement line
spacing were set to ~ 150 m and ~ 100 m, respectively, and the total flight distance was 191 km. The measured geomagnetic
total intensity was found to vary by ~ 1000 nT peak-to-peak. The estimated magnetization intensity derived from measured
data showed a 100 m thick magnetized surface layer with
2500
2500
North
South
normal polarity, composed of volcanic deposits of recent
Mt. Motoshirane
Yugama crater lake
2250
2250
activities. Underneath, a reverse-polarity magnetization
2000
2000
1750
1750
was found, probably corresponding to the Takai lava flow
1500
1500
in the Early Quaternary period (~ 1 Ma) mapped in the
1250
1250
region. Our results demonstrate the cost-effectiveness−3000 −2500 −2000 −1500 −1000 −500
0500 1000 1500 2000 2500 3000
and accuracy of using drone magnetometers for mapping
Northing [m]the rugged terrain of volcanoes.
−5

Keywords: Kusatsu-Shirane volcano, Aeromagnetic

−4

−3

−2

−1

0

1

2

3

magnetization intensity model [A/m]survey, Unmanned helicopter

Graphical abstract*Corresponding author: Takao Koyama, tkoyama@eri.u-tokyo.ac.jp
14

4

5

Earth, Planets and Space

EXPRESS LETTER

Open AccessTotal mass estimate of the January 23, 2018, phreatic eruption ofKusatsu-Shirane Volcano, central Japan
Nobuko Kametani*, Yasuo Ishizaki, Mitsuhiro Yoshimoto, Fukashi Maeno, Akihiko Terada,
Ryuta Furukawa, Ryo Honda, Yoshihiro Ishizuka, Jiro Komori, Masashi Nagai and Shinji Takarada
Earth, Planets and Space 2021, 73:141 DOI: 10.1186/s40623-021-01468-3
Received: 27 January 2021, Accepted: 22 June 2021, Published: 8 July 2021

Abstract
On January 23, 2018, a small phreatic eruption (VEI = 1) occurred at the Motoshirane Pyroclastic Cone Group in the southernpart of Kusatsu-Shirane Volcano in central Japan. The eruption ejected ash, lapillus, and volcanic blocks from three newly
opened craters: the main crater (MC), west crater (WC), and south crater (SC). Volcanic blocks were deposited up to 0.5 km
from each crater. In contrast, the ash released during this eruption fell up to 25 km ENE of the volcano. The total mass of thefall deposit generated by the eruption was estimated using two methods, yielding total masses of 3.4 × 104 t (segment
integration method) and 2.4 × 104 t (Weibull fitting method). The calculations indicate that approximately 70% of the fall
deposit was located within 0.5 km of the craters, which was mainly attributed to the low height of the eruption plume.

Keywords: Kusatsu-Shirane Volcano, Phreatic
eruption, Fall deposit, Isomass
contour map, Mass estimate

Graphical abstract*Corresponding author: Nobuko Kametani, kame@mfri.pref.yamanashi.jp

15

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Contents
Special issue “Understanding phreatic eruptions - recent observations of Kusatsu-Shirane volcano and equivalents -”. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yasuo Ogawa, Takeshi Ohba, Tobias P. Fischer, Mare Yamamoto and Art Jolly
A quest for unrest in multiparameter observations at Whakaari/White Island volcano, New Zealand 2007–2018. . . . . . . . . . Corentin Caudron, Társilo Girona, Arthur Jolly, Bruce Christenson, Martha Kane Savage, Roberto Carniel, Thomas Lecocq,
Ben Kennedy, Ivan Lokmer, Alexander Yates, Ian Hamling, Iseul Park, Geoff Kilgour and Agnès Mazot

Oxidation during magma mixing recorded by symplectites at Kusatsu–Shirane Volcano, Central Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kenta Ueki, Mutsuko Inui, Kenta Matsunaga, Naoya Okamoto and Kazuki Oshio
Classification of long-term very long period (VLP) volcanic earthquakes at Whakaari/White Island volcano, New Zealand. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Iseul Park, Arthur Jolly, Ivan Lokmer and Ben Kennedy
Identification of infrasonic and seismic components of tremors in single-station records: application to the 2013 and2018 events at Ioto Island, Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aika K. Kurokawa and Mie Ichihara
Locating hydrothermal fluid injection of the 2018 phreatic eruption at Kusatsu-Shirane volcano with volcanic tremor
amplitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Taishi Yamada, Aika K. Kurokawa, Akihiko Terada, Wataru Kanda, Hideki Ueda,
Hiroshi Aoyama, Takahiro Ohkura, Yasuo Ogawa and Toshikazu Tanada

Volcanic unrest at Hakone volcano after the 2015 phreatic eruption: reactivation of a ruptured hydrothermal system?
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kazutaka Mannen, Yuki Abe, Yasushi Daita, Ryosuke Doke, Masatake Harada,
George Kikugawa, Naoki Honma, Yuji Miyashita and Yohei Yukutake

Behavior of magmatic components in fumarolic gases related to the 2018 phreatic eruption at Ebinokogen Ioyama volcano,
Kirishima Volcanic Group, Kyushu, Japan . . . . . . Takeshi Ohba, Muga Yaguchi, Urumu Tsunogai, Masanori Ito and Ryo Shingubara
Response of a hydrothermal system to escalating phreatic unrest: the case of Turrialba and Irazú in Costa Rica (2007–2012). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Rouwet, R. Mora-Amador, C. Ramírez, G. González, E. Baldoni, G. Pecoraino,
S. Inguaggiato, B. Capaccioni, F. Lucchi and C. A. Tranne

Magma reservoir beneath Azumayama Volcano, NE Japan, as inferred from a three-dimensional electrical resistivity
model explored by means of magnetotelluric method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Masahiro Ichiki, Toshiki Kaida, Takashi Nakayama, Satoshi Miura, Mare Yamamoto,
Yuichi Morita and Makoto Uyeshima

The 2018 phreatic eruption at Mt. Motoshirane of Kusatsu–Shirane volcano, Japan: eruption and intrusion ofhydrothermal fluid observed by a borehole tiltmeter network. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Akihiko Terada, Wataru Kanda, Yasuo Ogawa, Taishi Yamada, Mare Yamamoto, Takahiro Ohkura,
Hiroshi Aoyama, Tomoki Tsutsui and Shin’ya Onizawa

1

6
6
7
7

8

8
9

9

10

10

Modeling long-term volcanic deformation at Kusatsu-Shirane and Asama volcanoes, Japan, using the GNSS coordinate timeseries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hiroshi Munekane 11
Crisis hazard assessment for snow-related lahars from an unforeseen new vent eruption: the 2018 eruption ofKusatsu-Shirane volcano, Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kyoko S. Kataoka, Kae Tsunematsu, Takane Matsumoto,Atsushi Urabe and Katsuhisa Kawashima

11

Coeruptive and posteruptive crustal deformation associated with the 2018 Kusatsu-Shirane phreatic eruption basedon PALSAR-2 time series analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yuji Himematsu, Taku Ozawa and Yosuke Aoki 12
On data reduction methods for volcanic tremor characterization: the 2012 eruption of Copahue volcano, Southern Andes. . . . . . . . . . . . . . . . . . . . . . . Ivan Melchor, Javier Almendros, Roberto Carniel, Kostas I. Konstantinou, Marcia Hantusch and Alberto Caselli 12
Anatomy of active volcanic edifice at the Kusatsu–Shirane volcano, Japan, by magnetotellurics: hydrothermal implications
for volcanic unrests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kuo Hsuan Tseng, Yasuo Ogawa, Nurhasan, Sabri Bülent Tank, Naoto Ujihara,
Yoshimori Honkura, Akihiko Terada, Yoshiya Usui and Wataru Kanda

13

Reconstructing surface eruptive sequence of 2018 small phreatic eruption of Iwo-yama volcano, Kirishima Volcanic
Complex, Japan, by infrasound cross-correlation analysis . . . . . . . . . . . . . Dan Muramatsu, Takeshi Matsushima and Mie Ichihara 13
Kusatsu-Shirane volcano eruption on January 23, 2018, observed using JMA operational weather radars . . . . . . . Eiichi Sato 14
Aeromagnetic survey in Kusatsu-Shirane volcano, central Japan, by using an unmanned helicopter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Takao Koyama, Wataru Kanda, Mitsuru Utsugi, Takayuki Kaneko, Takao Ohminato, Atsushi Watanabe,
Hiroshi Tsuji, Taro Nishimoto, Alexey Kuvshinov and Yoshiaki Honda

Total mass estimate of the January 23, 2018, phreatic eruption of Kusatsu-Shirane Volcano, central Japan. . . . . . . . . . . . . . . . . . . . . . . . . . . . Nobuko Kametani, Yasuo Ishizaki, Mitsuhiro Yoshimoto, Fukashi Maeno, Akihiko Terada, Ryuta Furukawa,
Ryo Honda, Yoshihiro Ishizuka, Jiro Komori, Masashi Nagai and Shinji Takarada

14

15

Cover image: Terada, A. et al., Earth, Planets and Space 2021, 73:157 DOI: 10.1186/s40623-021-01475-4